The present invention relates to electronic circuits which have fusible links. Specifically, a circuit is described which enables a redundant circuit element in response to the ablation of a fusible link, and which inhibits dendritic growth on an ablated fusible link avoiding reconnection of the link.
Integrated circuit technology such as high density memories, including static random access memories and dynamic random access memories have many storage locations. High density integrated circuits are fabricated as an individual component where the entire device relies upon every circuit element. The entire memory may have to be discarded in the event of a failure to read or write to any storage location as a result of a single malfunctioning circuit element.
As a result of this consequence many semiconductor memories, as well as other circuits, are fabricated with redundant circuit elements. In the event of a failure of a circuit element, the redundant element may be substituted for the failed element. The redundant elements are enabled by ablating a fusible link which disables the failed component, and at the same time, activates the spare device. The process of ablating the fusible links is accomplished using a laser beam ablating the fusible link.
In other applications, fusible links are utilized to customize the chip function after manufacture. They may be used to customize the operations of a general purpose microcontroller, or to specify a determined optimal operating supply voltage. The fusible links may be used to provide an identification number for a circuit device to thwart theft or for secure communication from arrays for lookup tables.
Fusible links are extensively used in analog applications to trim the value of precision components, such as a capacitor or resistor, or to reduce the offset in amplifiers and comparators, as well as to finely trim component values of connected subassemblies to match output-input characteristics of the connected subassemblies.
The process of activating a redundant circuit element begins when the fusible link is ablated or "blown". Ablation is effected by laser radiation of the fusible link thereby vaporizing the link, or by forcing an excessive current through a very small diameter fusible link which heats to a temperature high enough to vaporize the link. The process of ablating a fusible link results in rupture of the dielectric encapsulation used on the integrated circuit and a portion of the metal material escapes as a vapor. The rupture in the dielectric passivation exposes the metal to unwanted chemical reactions. The ablation may also have another unfortunate consequence of rupturing any anticorrosive encapsulation material which may also be around the fusible link.
The fusible links are usually contained in the top most metal layer of the integrated circuit to provide easy access for laser ablation, and to permit the vaporized metallic material created during the ablation process to escape. In the past, the fusible links were typically aluminum, and when the fusible link is ablated or "blown", an oxide forms around the free ends of the fusible link largely self passivating the link against corrosion. In the future, however, integrated circuits will use other types of metal wiring, such as copper, which if implemented as a fusible link, will not produce a self passivating oxide. The free copper ends of a blown fusible link are subject to dendritic growth, which can reconnect the ends of a blown fusible link with a low enough resistance to reactivate the defective circuit component. The formation of dendritic growth on the ends of a conductor of a blown fuse is enhanced by the presence of a d.c. potential across the ends, as well as temperature and humidity. The exposed ends of a copper fusible link are also subject to corrosion which can proceed from the ruptured site and follow the wire material of the fusible link to a circuit junction where it may interrupt a conductive structure in the circuit wiring. Before integrated circuits may be implemented with metals other than aluminum, such as copper which are not self passivating, a solution to the forgoing problem is necessary.